doc: guide for runtime-specific conditional exports using --import

doc/contributing/runtime-conditional-exports.md

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+# Runtime-Specific Conditional Exports in Node.js
+
+Node.js supports conditional exports based on environments such as `"node"`, `"browser"`, and `"import"`. However, runtime-specific conditions like `"electron"` are not included by default and **currently cannot be added dynamically** to the module resolution process.
+
+This guide demonstrates how to simulate runtime-specific behavior without modifying Node.js core.
+
+## Example: Supporting `"electron"` as a Condition
+
+You can use the `--import` flag to preload a script that sets runtime-specific flags.
+
+### Step 1: Preload Script
+
+```js
+// register-conditions.js
+if (process.versions.electron) {
+  process.env.EXPORTS_CONDITION = 'electron';
+}
+```
+### Step 2: Application Code
+
+```js
+// index.js
+if (process.env.EXPORTS_CONDITION === 'electron') {
+  module.exports = require('./electron-specific.js');
+} else {
+  module.exports = require('./default.js');
+}
+```
+### Step 3: Run with Preload
+
+```bash
+node --import ./register-conditions.js index.js
+```
+
+### Notes
+
+- This technique does not affect the `exports` field resolution in `package.json`, but allows dynamic control within your application.
+- For advanced usage with ESM resolution, consider using a custom loader.
+